Saturation Binding of Nicotine to Synthetic Neuromelanin Demonstrated by Fluorescence Spectroscopy

Neuromelanin (NM) has long been considered as an aging pigment, perhaps an unavoidable and undesirable byproduct of dopaminergic neural transmission. However, NM is carefully packaged into double membrane-bound structures within cells of the substantia nigra and other neural tissues, suggesting a beneficial function to maintaining these stores. It is well established that NM is able to concentrate toxic xenobiotics within pigmented cells due to its unique chemical environment. In doing so, such agents may confer susceptibility to Parkinson's disease (PD) as illustrated by model PD-inducing neurotoxins such as methyl-phenyl-pyridinium ion. It is possible that high-affinity binding interactions toward NM may contribute to the adverse effects of PD-inducing toxins, as well as neuroprotective agents. Here we aim to develop a generalized assay capable of elucidating the binding constants of chemical agents to synthetic and natural neuromelanins. Toward this end, a model neuromelanin synthesized from dopamine and cysteine was prepared according to published procedure. Using a UV/Visible spectroscopic assay, we show that dopamine, 6-hydroxy dopamine, and nicotine bind to the synthetic neuromelanin, while caffeine did not. More importantly, nicotine was further found to induce a fluorescence signal in the presence of NM which was used to establish a binding constant estimated at 0.65 mM. Dopamine appears to enhance this signal, also in a saturable manner, with an estimated Kd of 0.05 mM in our isolated chemical system. In summary, the micro-scale fluorescence assay described herein will allow us to overcome many of the problems inherent in the study of chemical interaction with NM through traditional spectroscopic means. Using a single standardized signal, it should now be possible to rank a number of PD-related toxins based on NM-binding affinity and shed further light on this important problem.